The present invention relates generally to multilayer, tubular films and to methods and apparatus for making such films. More particularly, the invention relates to tubular, polymer films containing a barrier layer and having uniform barrier properties around the circumference of the tube.
Film structures including a barrier layer such as polyvinylidene chloride have been difficult to extrude in tubular form. Extrusion of polyvinylidene chloride presents special problems. One problem results from the thermal degradation of the polymer which can occur in the extrusion die. To overcome this, polyvinylidene chloride has been encapsulated in an encapsulating material which does not exhibit the thermal degradation of polyvinylidene chloride.
Another problem which arises in the formation of polyvinylidene chloride into a tubular film is the seam which is formed. The seam extends along a weld line on one side of the extrudate. Generally, the extrusion die used to produce a tubular extrudate defines an annular chamber into which the extrusion material is forced. The material separates into two substreams which flow in opposite directions around the annular chamber, meeting on the opposite side of the chamber where they recombine. The streams then exit from the annular opening, and define a weld line at the point of recombination.
If an encapsulated layer of a barrier material such as polyvinylidene chloride is extruded in this way, the polyvinylidene chloride core does not recombine along the weld line. Only the encapsulating material recombines at the weld line. Since the encapsulating material has a much higher gas transmission rate than polyvinylidene chloride, the tubular film has higher gas transmission properties at the weld line than in the rest of the tubular film. This is unacceptable in many applications.
U.S. Pat. No. 4,643,927, to Luecke et al., which is incorporated herein by reference, suggests one solution to this problem. Luecke discloses a multilayer film having a central layer of barrier material which overlaps itself by a substantial distance along the weld line. The patent states that an overlap of two thirds of one inch in the barrier layer along the weld line is sufficient to provide a film in which the oxygen transmission rate along the weld line is no greater than in other portion of the film.
While Luecke represents a significant improvement in the manufacture of tubular barrier films, problems still remain. Blown films containing polyvinylidene chloride can only be produced on small dies (those having a diameter of less than about 8 inches).
Furthermore, even these small blown film dies can only be operated for about 1 to 4 weeks before the line must be shut down and cleaned. Because of its thermal degradability, polyvinylidene chloride has a tendency to xe2x80x9ccarbonizexe2x80x9d in the extrusion equipment. Carbonization results in the formation of small carbon particles in the molten extrudate. Blown film dies have a large surface area where the molten polymer is exposed to long residence time, and polyvinylidene chloride has a tendency to adhere to the metal. The long residence time results in degradation of the polyvinylidene chloride. Black, degraded polymer may form, which can then break loose and contaminate the film. This is an even bigger problem on large dies (those having a diameter of greater than about 8 inches) due to the increased surface area and higher metal temperature as a result of higher temperature skin polymers conducting heat to the die mandrel. The carbon build-up requires the manufacturer to shut down and clean the extrusion apparatus. The shutdown and cleaning of the extrusion apparatus results in high maintenance costs and lost production time.
Thus, it would be desirable to make a coextruded blown film containing a barrier material, to produce such a film on large dies, and to operate for long periods of time without shutdowns due to carbon formation.
These needs are met by the tubular, multilayer film, methods and apparatus of the present invention. The tubular, multilayer film includes a central barrier layer and a pair of adhesive layers on opposite sides of the central barrier layer. The adhesive layers completely cover the central barrier layer. Opposing edges of the central barrier layer overlap longitudinally along the tubular, multilayer film. The total thickness of the central barrier layers in the overlapping portion is substantially the same as the thickness of the central barrier layer in the non-overlapping portion. The tubular, multilayer film also includes inner and outer surface layers. The inner surface layer extends completely around the interior of the tubular, multilayer film, and the outer surface layer extends completely around the exterior of the tubular, multilayer film. This arrangement covers the encapsulated barrier layer and protects it from degradation. Additional layers may be included as needed.
The invention also involves a tubular film including a central barrier layer overlapping by at least an amount determined by Equation 1 along a weld line which extends longitudinally along the tubular film. The central barrier layer has substantially the same total thickness in the overlapping portion as in the non-overlapping portion. An inner adhesive layer and an outer adhesive layer are positioned on opposite sides of the central barrier layer. The adhesive layers completely encapsulate the central barrier layer. An inner surface layer is positioned inside the inner adhesive layer, and an outer surface layer is positioned outside the outer adhesive layer.
The central barrier layer is preferably made from a polymer selected from vinylidene chloride polymers and copolymers, ethylene vinyl alcohol polymers and copolymers, polyamide (Nylon) polymers and copolymers, and acrylonitrile polymers and copolymers. The adhesive layers are preferably made from a polymer selected from ethylene vinyl acetate (EVA) polymers and copolymers, ethylene methyl acrylate (EMA) polymers and copolymers, ethylene acrylic acid (EAA) polymers and copolymers, ionomers, and maleic anhydride grafted olefin polymers and copolymers. The surface layers are preferably made from a polymer selected from polyethylene polymers and copolymers, nylon and K-resins (styrene/butadiene block copolymers), ethylene vinyl acetate copolymer (EVA), polypropylene (PP) and polyethylene terephthalate (PET).
The present invention also includes a method of making a tubular, multilayer film. The method includes extruding a block of material having a barrier core and an adhesive covering the barrier layer core into a first stream having a generally annular cross-section. The first stream has a central barrier layer which overlaps longitudinally along the tubular, multilayer film, such that the total thickness of the central barrier layers in the overlapping portion is substantially the same as the thickness of the central barrier layer in the non-overlapping portion. An inner surface layer is extruded into a second stream having a generally annular cross-section. The second stream is positioned within the first stream and is joined thereto by the adhesive. An outer surface layer is extruded into a third stream having a generally annular cross-section. The third stream is positioned to surround the first stream and is joined thereto by the adhesive. The first stream is preferably extruded such that the opposing longitudinally extending edges of the central barrier layer overlap. The present invention also includes a method for coextruding a multilayer tubular film having a barrier material. A core extrudate of barrier material is extruded with a core extruder. A preencapsular extrudate of preencapsular material is extruded and directed to a preencapsulation die provided adjacent to the outlet of the core extruder. The core extrudate and the preencapsular extrudate are joined in the preencapsulation die in a coaxial relationship wherein the preencapsular extrudate is disposed radially outwardly of the core extrudate to form a preencapsulated core extrudate. An inner layer extrudate and an outer layer extrudate are extruded. The preencapsulated core extrudate is fed through a distribution manifold to a coextrusion die. The distribution manifold is designed to overlap opposing longitudinally extending edges. A multilayer blown film having the inner layer extrudate disposed radially inwardly of the preencapsulated core extrudate and the outer layer extrudate disposed radially outwardly of the preencapsulated core extrudate is formed. The coextrusion die has an annular channel adjacent to the distribution manifold to receive the preencapsulated core extrudate from the manifold channels. The depth of the annular channel is such that the flow of the polymer is not excessively restricted, and is preferably approximately twice the depth of the end of one manifold channel in the set distance.
The preencapsulation die preferably produces a preencapsulated core extrudate having non-uniform layer thicknesses. The preencapsulation die preferably has a first die gap and a second die gap, the first die gap being greater than the second die gap so that more polymer flows through the first die gap than through the second die gap.
The inner layer extrudate and the outer layer extrudate can be joined to the preencapsulated core extrudate either before or after the preencapsulated core extrudate is fed through the coextrusion die. Additional inner layers and outer layers can be included, if desired.
Another aspect of the invention is an extrusion apparatus for coextruding a multilayer film from a plurality of feed stock materials. The apparatus includes a core extruder for extruding a core extrudate, and a preencapsular extruder for extruding a preencapsular extrudate. A preencapsular transfer tube transfers the preencapsular extrudate to the preencapsulation die, which is disposed adjacent the outlet of the core extruder. A preencapsulated core extrudate transfer tube disposed downstream of the preencapsulation die transfers the preencapsulated core extrudate to the coextrusion die, which has a distribution manifold.
Another aspect of the invention is the distribution manifold. The distribution manifold includes a body having an inlet end and an outlet end, a manifold inlet at the inlet end of the body, and a pair of manifold channels. The pair of manifold channels has substantially the same length and extends from the manifold inlet around the body in opposite directions. Opposite ends of the manifold channels overlap each other by a set distance at a point opposite the manifold inlet. The opposite ends of the manifold channels are at different radial distances from the center of the body such that the preencapsulated core extrudate in the overlapping ends of the manifold channels remains separated. If the geometry is planar, the ends of the manifold channel will be at the same radial distance. The manifold channels decrease in the cross-sectional area from the manifold inlet to the opposite end. The manifold channels preferably have a streamlined shape, preferably a teardrop shape. The manifold channels preferably have an aspect ratio of height to depth of greater than 3:1. The distribution manifold preferably has a depression in the body located where the manifold channels overlap. The depression is at a first radial distance from the center of the body, and the set distance of the end of one manifold channel is located in the depression. There is an insert positioned over the depression. The insert is at a second radial distance from the center of the body, the second radial distance being greater than the first. The set distance of the end of the second manifold channel is located on the insert. The depression and the insert define a gap therebetween so that the preencapsulated core extrudate from the end of the manifold channel located in the depression flows through the gap.
Another aspect of the invention is a preencapsulation die for preencapsulating thermally sensitive polymer. The preencapsulation die includes a die body having an annular opening therethrough. The die body has a first member and a second member adjacent to the first member. The preencapsulation die includes an inner mandrel which extends circumferentially around the annular opening in the first member. The inner mandrel has a first surface and a second surface. The first surface is lower than the second surface whereby the first surface of the inner mandrel and the second member define a first die gap, and the second surface of the inner mandrel and the second member define a second die gap, the first die gap being greater than the second die gap. There is also a preencapsulation distribution manifold which extends circumferentially around the inner mandrel in the first member.
The preencapsulation die optionally includes a resin distribution channel extending about 180 degrees circumferentially around the preencapsulation distribution manifold in the first member. The resin distribution channel preferably terminates in an opening at each end. The openings in the resin distribution channel communicate with the preencapsulation distribution manifold. The resin distribution channel communicates with a resin inlet, which is located intermediate the openings in the resin distribution channel. The openings in the resin distribution channel are preferably positioned adjacent the first surface of the inner mandrel. The inner mandrel preferably has a pair of first surfaces and a pair of second surfaces. The first surfaces preferably extend approximately 60 degrees around the annular opening, and the second surfaces preferably extend approximately 120 degrees around the annular opening. The first surfaces are preferably positioned on opposite sides of the inner mandrel adjacent to the openings in the resin distribution channel, and the second surfaces are preferably positioned between the first surfaces on opposite sides of the inner mandrel.
The second member of the preencapsulation die can be flat. Alternatively, it could be a mirror image of the first member.